The object is achieved by the characteristics stated in the description below and in the following claims.
A single-acting, vertically mounted hydraulic cylinder of a type that is known per se has a protruding cylinder rod at the upper end of the cylinder. The piston rod is provided with a piston according to prior art. The piston rod lead-through through the gable wall of the cylinder, and the piston, are provided with suitable seals according to prior art.
Advantageously, the cylinder is supplied with oil through an axial cylindrical passage disposed centrally in the piston rod.
The cylinder is designed to lift a load when connected to a hydraulic system that is known per se and when hydraulic pressure is established in a space above a piston in the cylinder.
A bottom plate that fits tightly at a lower end of the cylinder is provided with a drainage channel. The drainage channel is connected, by means of techniques that are known per se, with a reservoir for collection of oil that is drained from a space between the cylinder piston and the bottom of the cylinder.
The bottom of the cylinder is provided with a shoulder for the piston, so as to leave a space between the piston and the cylinder bottom when the piston is pushed fully into the cylinder.
The bottom plate of the cylinder is provided with an air inlet that projects slightly from the bottom, at the most to the level of the piston shoulder in the cylinder bottom. The air inlet is connected, by means of techniques that are known per se, with a compressed-air plant that is known per se.
When using the cylinder of the invention, hydraulic pressure is introduced at the top of the cylinder piston. Over time, oil will leak between the cylinder wall and the piston seals. The leaking oil collects at the bottom of the cylinder, from where it is drained via said drainage channel. By using said compressed-air plant, the space between the piston and the cylinder bottom is pressurized to a slight overpressure, typically 1.5 bar. As a result of this overpressure the leaking oil will drain to a reservoir which may be placed in a, for this, favourable location high above the cylinder, e.g. on a deck of an oil installation.